Kristen was born and raised in Los Angeles, California. Her father, John Stewart works as a stage manager and television producer. Kristen's mother, Jules Mann-Stewart, is a script supervisor and was born in Australia. Kristen has an older brother, Cameron Stewart.

Kristen started acting when she was eight years old, after an agent saw her perform at her school's Christmas event. Her first minor roles were in programs such as The Thirteenth Year and The Safety Of Objects and her first major role was in a movie called Panic Room. Since then, Kristen has appeared in films such as Cold Creek Manor, Catch That Kid, Undertow and Speak.